The present invention relates generally to compressors, and more particularly, provides novel apparatus and methods for preventing compressor surge.
A particularly difficult problem arising in the design and operation of compressors used in gas turbine engines is the prevention of surge--a condition commonly defined as the lower limit of stable operation of a compressor, and generally comprising the undesirable reversal of fluid flow through the compressor which oftentimes causes damaging pulsation therein. In the past, a variety of solutions to this surging problem have been proposed. However, for a number of reasons, such as complexity, inefficiency, or simply the relatively high cost of such prevention techniques, none of these conventional approaches to surge prevention has proven entirely satisfactory.
As an example, several conventional surge systems use an open loop scheduling technique that typically requires inputs from a number of parameters such as inlet guide vane position, bleed air total pressure, bleed air static pressure, engine speed, altitude, electrical load and the like. The use of these numerous control parameters typically results in expensive sensors, reliability degradation, and complex logic circuitry to eliminate compressor surging. Precise overall control accuracy is difficult to achieve with such conventional systems due to a combination of sensor inaccuracies and the actual definition of the particular compressor's surge line. Since these particular surge prevention systems are of the open loop type, a "worst case" surge control line must be used that will be suitable for all engines in which the particular compressor would be used. This worst case design necessity results in wasted surge bleed flow and higher than necessary overall fuel consumption. Additionally, various of the sensors required to generate output signals indicative of these parameters must, of necessity, be positioned in the compressor's air flow path. This, of course, tends to interfere with the compressor's overall aerodynamic efficiency.
Another conventional approach, employed in the compressor's diffuser section, is to tilt one of the diffuser vanes relative to the other vanes so that the tilted vane's incidence angle is slightly increased relative to the other vanes. As the compressor approaches a surge condition, the tilted vane begins to stall as air flow begins to separate therefrom. Upon sensing this single vane stall condition, the surge control system automatically reduces fuel flow to the turbine engine to prevent complete compressor stall. However, a significant disadvantage of this system is that the "tilted" vane actually increases the likelihood that the compressor will surge prematurely since the initial stall condition of any particular vane normally is the event that triggers surge. Additionally, this system is of necessity an "on-off" type which is considerably less desirable than a modulating type control system.
Still other conventional systems attempt to avoid compressor surge by utilizing complicated and relatively expensive fuel scheduling systems which automatically change the engine fuel flow to avoid or bypass operating regions of the compressor in which surge may occur. These fuel control-oriented systems are typically characterized by high cost, operating inefficiency, and increased overall fuel consumption.
Accordingly, it is an object of the present invention to provide apparatus and associated methods for preventing compressor surge which eliminate or minimize above-mentioned and other problems.